The number of swans on England's River Thames plummeted by around 40 percent during the past year, according to a survey of the majestic white birds that has been carried out annually since the Middle Ages.

The survey, which is known as a "swan upping", is conducted along 79 miles (127 kilometers) of the river with much pomp and ceremony, owing to the fact that all swans in the United Kingdom are considered to be the personal property of the monarch.

This year, the count found the population of young birds, known as cygnets, had been severely dented by an outbreak of avian flu.

The man in charge of the count, David Barber, said an increasingly large number of the goose-like birds are also being killed each year by dogs, and in deliberate acts of cruelty carried out by people. Barber said swans lay their eggs in nests on the ground, next to the river, and several of them were washed away this year in floods, contributing the decline.

"Youths with air rifles and catapults see a big white target to shoot," he told The Times newspaper. "Dog walkers will let their dog off the lead along the riverbank. They're attacked and killed."

Barber, who goes by the official title of the king's swan marker, told the BBC he and his team only located 94 young swans this year during their five-day search along the river, which is England's longest and which is the main waterway through the capital, London.

He said the fall in the population was "horrendous, terrible to see".

The total of 94 swans that were caught, weighed, ringed, and released this year was down from the 155 counted in 2022.

With avian flu thought to be the main reason for the fall in numbers, the National Health Service warned that the disease can also infect humans in rare cases, and said people should avoid close contact with birds and their droppings. The UK has recorded 190 confirmed cases of highly pathogenic avian influenza among swans since Oct 1, 2022. The flu is thought to have killed around 50,000 wild birds in the UK since 2021.

The annual swan upping survey, with its red-coat-clad counters and use of traditional rowing boats, serves a serious environmental function as well as a historical one, giving the authorities a clue about the health of England's waterways and their inhabitants.

However, the survey began in 1186 as a way for the monarch to make sure no one else was eating his swans, which were considered a delicacy fit only for a king and his cronies.

More than eight hundred years later, the birds are a protected species and no one can legally kill or eat them.